The increasing demand for computer performance has led to higher chip internal clock frequencies and parallelism, and has increased the need for higher bandwidth and lower latencies. Processor frequencies are predicted to reach 29 GHz by 2018, and off-chip signaling interface speeds are expected to exceed 56 Gb/s. Optimization of bandwidth, power, pin count, or number of wires and cost are the goals for high-speed interconnect design. The electrical performance of interconnects is restricted by noise and timing limitations of the silicon, package, board and cable. To that end, semiconductor packages must be made smaller, conforming more and more closely to the size of the die encapsulated within. However, as the size of the package shrinks to the size of the die itself, the size of the package becomes insufficient to support the number of leads generally required by current applications.
Chip Scale Packages (CSP) have emerged as the dominant package for such applications. FIG. 1 shows an example of a CSP in current practice. More specifically, the package in FIG. 1 is a Wafer Level Chip Scale Package 10 (WLCSP), commonly marketed by companies such as National Semiconductor Corporation as the Micro SMD and Maxim Integrated Products as the UCSP. Generally, solder bumps 11 are formed on processed and completed semiconductor wafers 12 before the wafers are sawed to form an individual semiconductor device 13. Although this has dramatically reduced package size and can be useful in some instances, it suffers from drawbacks which remove it from consideration for certain applications. First, the pitch between the solder bumps 11 must be made wide enough to effectuate assembly of the device onto a printed circuit board in application. This requirement can force manufacturers to artificially grow die sizes to meet the minimum pitch, thereby increasing cost. Second, the total I/O count of the device is generally constrained due to the decreased reliability at the high bump counts. At bump counts higher than 49, or a 7×7 array, reliability becomes critical and applications such as hand held devices, which require a high degree of reliability, no long become a possible marketplace.
To overcome the issues mentioned above, the semiconductor industry has moved toward Ball Grid Array (BGA) packages. The BGA is descended from the pin grid array (PGA), which is a package with one face covered (or partly covered) with pins in a grid pattern. These pins are used to conduct electrical signals from the integrated circuit (IC) to the printed circuit board (PCB) it is placed on. In a BGA, the pins are replaced by balls of solder formed on the bottom of the package. The device is placed on a PCB that carries copper pads in a pattern that matches the solder balls. The assembly is then heated, such as in a reflow oven or by an infrared heater, causing the solder balls to melt. Surface tension causes the molten solder to hold the package in alignment with the circuit board at the correct separation distance while the solder cools and solidifies. The BGA is a solution to the problem of producing a miniature package for an IC with many hundreds of I/O. As pin grid arrays and dual-in-line (DIP) surface mount (SOIC) packages are produced with more and more pins, and with decreasing spacing between the pins, difficulties arose in the soldering process. As package pins got closer together, the danger of accidentally bridging adjacent pins with solder grew. BGAs do not have this problem, because the solder is factory-applied to the package in exactly the right amount. Alternatively, solder balls can be replaced by solder landing pads, forming a Land Grid Array (LGA) package.
FIG. 2 shows a cutaway image of a generic BGA package 20. Generally, an IC 21 has bondpads 22 to which bondwires 23 are affixed. The IC 21 is mounted on a substrate 24. In current practice, the substrate 24 is a laminate, such as polyimide. Generally, the substrate 24 is of a similar construction to a PCB. The substrate 24 has copper patterns 25 formed thereon. The bondwires 23 effectuate electrical contact between the IC 21 and the copper patterns 25. The copper patterns 25 are electrically connected to solder balls 26 through via holes 27 in the substrate 24. In most embodiments of BGA packages, the IC 21 is encapsulated by a mold compound 28. Although BGA packages effectuate large I/O count devices in small areas, they are susceptible to moisture. Generally, moisture seeps into packages while awaiting assembly into a finished product, such as a computer. When the package is heated to solder the device into its end application, moisture trapped within the device turns into vapor and cannot escape quickly enough, causing the package to burst open. This phenomenon is known as the “popcorn” effect. What is needed is a semiconductor package that is robust to both structural stressors and moisture.